[llvm] [TailRecElim] Handle discarded-call return conflicts with a shift accumulator (PR #221694)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto commented:

Looks reasonable, thanks!

(Per the [DeveloperPolicy](https://llvm.org/docs/DeveloperPolicy.html#commit-messages), we usually prefer not to tag people in PR description / commit message, so as to avoid receiving spurious notifications on cherry-picks.)

https://github.com/llvm/llvm-project/pull/221694